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Polyommatus icarus
Plombières.
Many thanks bayucca, for taking time in IDing this butterfly! With these I never dare to give an ID myself anymore, as they are so variable. I suggested Lycaena or Lysandra because they figure as species found in Plombières but the Polyommatus is also certainly here in Belgium so you must be right ;-)
Usually not so easy to determine from only a ventral view, but in my eyes yours is Polyommatus icarus.
http://www.natur-schmetterlinge.ch/schme...
http://www.pyrgus.de/Polyommatus_icarus....
http://www.schmetterling-raupe.de/art/ic...